在 onResume() 中如何获取 View 宽高

###一、测试代码
1.activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.babycy.getviewwh.MainActivity">

    <Button
        android:id="@+id/btn"
        android:layout_width="100dp"
        android:layout_height="40dp" />

</RelativeLayout>

2.MainActivity.java

public class MainActivity extends AppCompatActivity {

    private Button mBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mBtn = (Button) findViewById(R.id.btn);

        Log.d("TAG", "onCreate() button width=" + mBtn.getWidth());
    }

    @Override
    protected void onResume() {
        super.onResume();

        mBtn.post(new Runnable() {
            @Override
            public void run() {
                Log.d("TAG", "onResume() mBtn post button width=" + mBtn.getWidth());
            }
        });

        new Handler().post(new Runnable() {
            @Override
            public void run() {
                Log.d("TAG", "onResume() Handler button width=" + mBtn.getWidth());
            }
        });
    }
}

打印结果：

###二、分析步骤
我们先大致了解下 Activity 的启动流程（生命周期）：

在上图的最下方的 scheduleLaunchActivity() 接口方法之后的流程：handleLaunchActivity() -> handleResumeActivity() -> performResumeActivity() -> Activity#performResume() -> Instrumentation#callActivityOnResume() -> Activity#onResume()

我们知道在 Activity 可见之后，界面绘制就要被触发了，最终调用的是 Activity 的 onResume() 方法

handleResumeActivity() {
	//...
	r = performResumeActivity(token, clearHide, reason);//调用 onResume() 方法
	//...
	wm.addView(decor, l);//对 mAttachInfo 进行赋值
	//...
}

可以看出 onResume() 方法在 addView() 方法前调用

重点关注：onResume() 方法所处的位置，前后都发生了什么？
从上面总结的流程看出，onResume() 方法是由 handleResumeActivity 触发的，而界面绘制被触发是因为 handleResumeActivity() 中调用了wm.addView(decor, l);

public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
    //...
    
    ViewRootImpl root;
    View panelParentView = null;

    synchronized (mLock) {
        //...

        root = new ViewRootImpl(view.getContext(), display);

        view.setLayoutParams(wparams);

        mViews.add(view);
        mRoots.add(root);
        mParams.add(wparams);
    }

    // do this last because it fires off messages to start doing things
    try {
	    //触发开发绘制，参考 1
        root.setView(view, wparams, panelParentView);
    } catch (RuntimeException e) {
        //...
        throw e;
    }
}

参考 1：setView()

/**
 *We have one child
 */
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            mView = view;

            //...

            // Schedule the first layout -before- adding to the window
            // manager, to make sure we do the relayout before receiving
            // any other events from the system.
            //触发界面刷新，参考 2
            requestLayout();
            if ((mWindowAttributes.inputFeatures
                    & WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
                mInputChannel = new InputChannel();
            }
            //...
            try {
                mOrigWindowType = mWindowAttributes.type;
                mAttachInfo.mRecomputeGlobalAttributes = true;
                collectViewAttributes();
                res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                        getHostVisibility(), mDisplay.getDisplayId(),
                        mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                        mAttachInfo.mOutsets, mInputChannel);
            } catch (RemoteException e) {
                //...
                throw new RuntimeException("Adding window failed", e);
            } finally {
                if (restore) {
                    attrs.restore();
                }
            }

			//...

			//这里的 view 是 DecorView
            view.assignParent(this);
            //...
        }
    }
}

参考 2：requestLayout()

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        //准备刷新，参考 3
        scheduleTraversals();
    }
}

参考 3：scheduleTraversals()

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        //设置同步障碍 Message
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        //屏幕刷新信号 VSYNC 监听回调把 mTraversalRunnable（执行doTraversal()）push 到主线程了，异步 Message 会优先得到执行 ，具体看下 Choreographer 的实现
        //mTraversalRunnable，参考 4
        mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

参考 4：mTraversalRunnable

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
	    //参考 5
        doTraversal();
    }
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

参考 5：doTraversal()

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        //移除同步障碍 Message
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }

		//参考 6，在上面移除同步障碍后，开始对控件树进行测量、布局、绘制
        performTraversals();

        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

参考 6：performTraversals()

private void performTraversals() {
	// cache mView since it is used so much below...
	final View host = mView;

	//...

	Rect frame = mWinFrame;
	if (mFirst) {
		mFullRedrawNeeded = true;
		mLayoutRequested = true;

		//...

		// We used to use the following condition to choose 32 bits drawing caches:
		// PixelFormat.hasAlpha(lp.format) || lp.format == PixelFormat.RGBX_8888
		// However, windows are now always 32 bits by default, so choose 32 bits
		mAttachInfo.mUse32BitDrawingCache = true;
		mAttachInfo.mHasWindowFocus = false;
		mAttachInfo.mWindowVisibility = viewVisibility;
		mAttachInfo.mRecomputeGlobalAttributes = false;
		mLastConfiguration.setTo(host.getResources().getConfiguration());
		mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;
		// Set the layout direction if it has not been set before (inherit is the default)
		if (mViewLayoutDirectionInitial == View.LAYOUT_DIRECTION_INHERIT) {
			host.setLayoutDirection(mLastConfiguration.getLayoutDirection());
		}
		host.dispatchAttachedToWindow(mAttachInfo, 0);
		mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
		dispatchApplyInsets(host);
		//Log.i(mTag, "Screen on initialized: " + attachInfo.mKeepScreenOn);

	} else {
		desiredWindowWidth = frame.width();
		desiredWindowHeight = frame.height();
		if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {
			if (DEBUG_ORIENTATION) Log.v(mTag, "View " + host + " resized to: " + frame);
			mFullRedrawNeeded = true;
			mLayoutRequested = true;
			windowSizeMayChange = true;
		}
	}

	//...

	// Execute enqueued actions on every traversal in case a detached view enqueued an action
	getRunQueue().executeActions(mAttachInfo.mHandler);
	
	//...
	
	performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
	performLayout(lp, mWidth, mHeight);
	performDraw();
	
	//...
}

####1. 在 onResume() 中 handler.post(Runnable) 获取不到 View 的真实宽高
原因：查看上面"参考 6：performTraversals()"执行的时机，因为在 scheduleTraversals() 中设置同步障碍消息，相当于在 MessageQueue 中插入了一个 Message，并且是插入到了 onResume() 之后，所以在 onResume() 中 handler.post(Runnable) 的异步消息会在同步障碍 Message 后立刻被 Handler 轮训找到后被处理，而此时界面还没有开始刷新绘制，待查询到同步障碍 Message 时候，会等待下个异步 Message（刷新Message）出现，所以在 onResume() 中 handler.post(Runnable) 中操作 UI 是失效的

####2. View.post(Runnable) 为什么可以获取到 View 的宽高？

public boolean post(Runnable action) {
	//mAttachInfo 是在 ViewRootImpl 的构造函数中初始化的
	//而 ViewRootmpl 的初始化是在 addView() 中调用
	//所以此处的 mAttachInfo 为空，所以不会执行该 if 语句
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.post(action);
    }

    // Postpone the runnable until we know on which thread it needs to run.
    // Assume that the runnable will be successfully placed after attach.
    //保存消息到 RunQueue 中，等到在 performTraversals() 方法中被执行
    getRunQueue().post(action);
    return true;
}

由参考 6 可知，我们通过 View.post(Runnable) 的 Message 会在 performMeaure() 之前被调用，那为什么还可以正确的获取到 View 的宽高呢？其实我们的 Message 并没有立即被执行，因为此时主线程的 Handler 正在执行的 Message 是 TraversalRunnable，而 performMeaure() 方法也是在该 Message 中被执行，所以排队等到主线程的 Handler 执行到我们 post 的 Message 时，View 的宽高已经测量完毕，因此我们也就很自然的能够获取到 View 的宽高